Today I discovered that in actual fact, I am a bigger part of the "issue" than I thought. He was heavy, flat and strung out, and I realized I was slouching. As soon as I realized this and straightened up, he got lighter and lifted his back. Thinking it HAD to be a coincidence, I slouched again, on purpose. He went heavy, flat and strung out, hollowed, and poked his nose. When I straightened up again, same result, MUCH lighter horse.
It is incredible how much the position of a rider's shoulders influences the horse's way of going! Don't get me wrong my gelding still gets heavy, flat and strung out at times, and still takes time to work down, but if I'm concentrating more on keeping a strong upper body and less on how he's actually working, he works so much better.
Next show I go to, I need to concentrate less on my horse, and more on RIDING my horse.